| The saddest part of the TMI accident is that the entire thing could have been avoided if TMI had modified its cooling system with the lessons learned from an identical series of events that happened two years earlier at an identical Babcock&Wilcox BWR in Ohio, except Davis-Besse was operating at 9% power instead of at 100% like TMI in Pennsylvania in 1979: https://www.nrc.gov/docs/ML1920/ML19208C067.pdf#page=4 "On September 24, 1977, Davis Besse Nuclear Power Station Unit No. 1 experienced a depressurization when a pressurizer power relief valve failed in the open position. The Reactor Coolant System (RCS) pressure was reduced from 2255 psig to 875 psig in approximately twenty-one (21) minutes. At the beginning of this event, steam was being bypassed to the condenser and the reactor thermal power was at 263 MW, or 9.5%. Electricity was not being generated. The following systems malfunctioned during the transient: a. Steam and Feedwater Rupture Control System (SFRCS). b. Pressurizer Pilot Actuated Relief Valve. c. No. 2 Steam Generator Auxiliary Feed Pump Turbine Governor" "At approximately 21 minutes into the transient, the operators discovered that the pressurizer power relief valve was stuck open. Blowdown via this valve was stopped by closing the block valve, thus terminating the reactor vessel depressurization. The RCS pressure recovered to normal and cooldown of the system followed." "The reason for the spurious 'half-trip' of the SFRCS has not yet been determined. An extensive investigation revealed several loose connections at terminal boards, but nothing conclusive. Investigation into the failure of the pressurizer pilot actuated relief valve revealed that a 'close' relay was missing from the control circuit. This missing relay would normally provide a 'seal-in' circuit which would hold the valve open until the pressure dropped to 2205 psig. Without the relay the power relief valve cycled open and closed each time the pressure of the RCS went above or below 2255 psig. The rapid cycling of the valve caused a failure of the pilot valve stem, and this failure caused the power relief valve to remain open." |